1. Field Of The Invention
The present invention relates to a method for collecting magnetizable particles located at the interface between a liquid product and water in an above ground storage tank. More specifically, the present invention relates to a method for collecting magnetic particles in the area of a petroleum-water interface in the storage tank where leaks are being detected by the sensing of changes in conductivity in a sensor assembly extending across the interface to prevent fouling of conductivity sensors of the sensor assembly with magnetizable particles.
2. Description of The Prior Art
Leaks in large above ground petroleum product storage tanks containing liquids such as regular gasoline, light fuel oil, diesel fuel and regular gasoline are a serious problem to the petrochemical industry from the standpoint of both inventory losses and environmental degradation.
The large size of such tanks makes leak detection difficult. In this respect, a leak rate of one barrel per hour in a 100 foot diameter tank will produce a level change rate of approximately 0.01 inches per hour and a direct measurement of the level change at the surface of the liquid in the tank is difficult due to the resolution required and to thermal expansion of the liquid, the tank and the detecting apparatus.
Furthermore, acoustic emission methods of leak detection previously utilized provide results which are clouded by the weakness of the leak as an acoustic source and by signal attenuation in the liquid or in the tank shell. Furthermore, background noise from wind and thermal expansion also appear to limit the reliability of leak detection determinations utilizing acoustic emissions.
Experience has shown that all significant leaks in above ground tanks occur in the area of the tank bottom. Also, experience has shown that thermal conditions near the bottom of the tank are stable. Furthermore, surface waves and convection currents in the liquid are effectively dampened by the time they reach the bottom of the tank.
A method for detecting leaks utilizes the environment adjacent the bottom of the tank for detecting leaks and the leak detector utilized in practicing this method is adapted to be positioned in the stable zone adjacent the bottom of the tank. With the method of the present invention, magnetizable particles at the interface between the liquid product in the tank and water in the tank are collected, preferably using a magnetic field established by magnets placed in the tank. This method can be practiced solely for the purpose of removing magnetizable particles from the interface between the liquid product and water in the tank and is preferably used in conjunction with the method for detecting leaks in the tank to prevent fouling of conductivity sensors of the leak detector situated in the tank.